This invention relates to a desulfurization catalyst comprising cobalt sulfate as an essential catalytic ingredient supported on a catalyst support. The catalyst permits the single-step conversion of sulfur oxides present as gases in stack gases and the like. By the term "sulfur oxides" we refer herein to both sulfur dioxide and sulfur trioxide. This one-step conversion of sulfur oxides, and particularly sulfur dioxide, to elemental sulfur which is not adsorbed on the catalyst but is discharged as elemental sulfur in the effluent from the catalyst bed, eliminates the conventional second-step regeneration of a `catalyst` bed and reduction of sulfur loaded upon conventional metal oxide acceptors, or in the alternative, of conversion of catalytically produced H.sub.2 S to elemental sulfur.
Conventionally, sulfur oxides are removed from gaseous mixtures such as stack or flue gases and smelter off-gases by contact with metal or metal oxide acceptors such as copper or copper oxide, respectively, on a refractory carrier material such as alumina. During contact, sulfur oxides are accepted by the metal or metal oxide, so that the purified gases, if discharged via a stack, cause substantially no air pollution. The metal sulfate, for example copper sulfate formed during acceptance, may be subsequently decomposd by means of reducing gas, the result being a regenerated acceptor and a sulfur dioxide-rich gas, which can be used, for example, to produce elemental sulfur or sulfuric acid. The regenerated acceptor can then be reused to purify a further quantity of gas containing sulfur oxides. In this two-step prior art process, the regeneration of the acceptor oxide, which is sometimes referred to as `catalyst`, is a difficult problem which often forms combustible deposits on the acceptor during the regeneration process. The combustible deposits are undesirable since their combustion during use of the regenerated acceptor causes a significant increase in temperature which adversely affects the acceptor life. More importantly, the two-step process requires that an inordinate expenditure of time be devoted to regeneration of the acceptor, the expenditure of which time is an economic deterrent.
Even in those instances where a single-step conversion of sulfur dioxide to elemental sulfur may be effected with an appropriate catalytic component suitably supported on a carrier, it has been found that the exotherms to which the catalyst is normally subjected, along with the reactions of the stack-gas components with the catalyst components, results in the decrepitation or disintegration of the catalyst so that a bed of catalyst soon develops so high a pressure drop as to become unusable. The problem of selecting a catalyst which is stable, has a desirable activity which may be supported on a support which will not interfere with the activity of the catalyst and yet defy attrition and decrepitation, at the same time permitting a conversion of sulfur dioxide to element sulfur in excess of 90 percent, has been a problem to which a great deal of effort has been devoted (see Removal of Sulfur dioxide from stack gases by Catalytic Reduction to Elemental Sulfur with Carbon Monoxide, Robert Querido and W. Leigh Short, "Ind. Eng. Chem. Process Des. Develop.", Vol. 12, No. 1, 1973). The catalyst of our invention is a solution to that problem.
U.S. Pat. No. 3,495,941 discloses a typical prior art desulfurization catalyst utilizing vanadium oxide supported on a carrier material. Also disclosed therein is cobalt molybdate which is disclosed for the reduction of sulfur dioxide with methane. In either case, sulfur dioxide is reduced to hydrogen sulfide which is thereafter converted to elemental sulfur.
Many chemical processes currently in commercial use employ catalysts which undergo a change in crystalline structure during the course of reaction. Such catalysts are particularly susceptible to attrition and other types of physical degradation. The desulfurization catalyst of our invention consists essentially of cobalt sulfate in crystalline form supported on an attrition resistant support such as gamma-alumina. However, formed gamma-alumina shapes do not maintain their physical strength, particularly at elevated temperature operation up to about 700.degree.C, and thus are not sufficiently decrepitation resistant. By `attrition resistant` catalyst is meant that the catalyst resists abrasion more or less, at or near the surface, while a general loss of strength of a shaped catalyst pellet is usually referred to as decrepitation or disintegration. Decrepitation of a catalyst pellet often permits it to be crushed by pressure between the thumb and forefinger. Numerous prior art catalysts have utilized clays of various types as an ingredient for a catalytic support. In most instances, the clay containing support is used as a binder and the catalyst support is thereafter fired to decompose the catalytic ingredient present in the form of a salt or hydroxide to the oxide form. In particular, U.S. Pat. No. 3,146,210 to Baldwin teaches the preparation of attrition-resistant alumina, beryllia, and zirconia catalyst pellets which can be used as catalyst supports by subsequent impregnation of the pellets with metallic salts. Attrition resistance and the maintenance of the physical strength of a supported catalyst are serious problems to which much attention has been devoted. Baldwin made no reference to the use of clay to enhance attrition and decrepitation resistance and thus, overlooked the discovery that clays can provide surprising transverse and crush strength to a tableted or pelleted alumina support.
The term "clay" when used in context with the present invention, is to be interpreted in the broadest sense and this invention is not limited by subtle differences and the composition of substances which were or could be classified in the broad sense as clays. Thus, a clay may be defined as "an earthy or stoney mineral aggregate consisting essentially of hydrous silicates and alumina, plastic when sufficiently pulverized and wetted, rigid when dry, and vitreous when fired at sufficiently high temperatures." Alternatively, a clay may be broadly defined as a "mixture of hydrous silicates of aluminum, iron, and magnesium with or without other rock and mineral particles, said clays being characterized by extreme fineness of particles (often colloidal in size) and by wide variations in physical and thermal (ceramic) properties and in mineral and chemical composition." Other definitions of the term "clay" may be found in the following volumes and the references contained therein and such clays are useful in the present invention:
Thorpes Dictionary of Applied Chemistry, J. F. Thorpe and M. A. Whiteley, Volume 3, Fourth Edition, Longmans, Green and Co., New York (1953)
Encyclopedia of Chemical Technology, Kirk-Othmer, Volume 6, Second Edition, Inter-Science publishers, New York (1965).
The preferred clays for use with our invention include: the kaolin group, including for example, kaolinite, dickite, nacrite, anauxite, halloysite, and endellite; the montmorillonite group, including for example, montmorillonite, beidellite, nontronite, hectorite, saponite, saucounite, and bentonite; the attapulgite and sepiolite group, including for example, attapulgite taken from the region of Attapulgus, Ga.; the high alumina clays, including for example, diaspore, boehmite, Gibbsite, and cliachite; and also the ball clays found principally in Kentucky and Tennessee and the fire clays produced in Missouri, Illinois, Ohio, Kentucky, Mississippi, Alabama, and Arkansas. Mixtures of the forementioned clays are likewise useful in the present invention as the clay portion of the binder.